Cellular networks are undergoing a shift from being purely voice centric to a mixture of voice and data in third generation (3G) systems and beyond. The networks are also evolving into more packet based wireless broadband access networks (WBANs). The WBANs are designed to support voice, data, and multi-media applications. These applications have varying requirements for the delay budgets, packet loss error rates, and priority, sometimes referred to as the quality of service (QOS) requirements.
Multiple Input Multiple Output (MIMO) or multiple antenna technology is one of the technologies proposed to be used in fourth generation (4G) wireless systems. MIMO technology can improve the reliability of the underlying wireless channel or increase the spectral efficiency using different configurations. For example, spatial multiplexing (SM) can be used to increase spectral efficiency by transmitting multiple parallel streams of data from multiple antennas, while diversity techniques, such as Space Time Block Codes (STBC) or Space Frequency Block Codes (SFBC) can be used to increase the channel reliability by intelligent coding across space and time or frequency, respectively. Similarly various modulation and channel coding scheme (MCS) levels can be used to obtain a tradeoff between reliability and spectral efficiency.
Hybrid automatic repeat request (HARQ) is another technology used in 4G systems to obtain reliable transmissions. However, an increase in the number of re-transmissions by HARQ leads to larger delays.